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A cooling apparatus includes: a circulation path of coolant; a first pump
provided in the circulation path, the first pump including a first inlet
and a first outlet; a second pump connected to the first pump, the second
pump including a second inlet and a second outlet; a first main pipe with
one end connected to the first outlet; a second main pipe with one end
connected to the second inlet; a connection portion connecting another
ends of the first and second main pipe; a first bypass pipe connecting
the first inlet and the connection portion; and a second bypass pipe
connecting the second outlet and the connection portion, wherein the
first main pipe and the second bypass pipe are connected in a same
direction in the connection portion, and the second main pipe and the
first bypass pipe are connected in the same direction.

1. A cooling apparatus comprising: a circulation path in which a coolant
circulates, where the coolant cooling an electronic component; a first
pump provided in the circulation path, the first pump including a first
inlet configured to take in the coolant and a first outlet configured to
discharge the coolant; a second pump connected to the first pump in
series in the circulation path, the second pump including a second inlet
configured to take in the coolant and a second outlet configured to
discharge the coolant; a first main pipe with one end that is connected
to the first outlet; a second main pipe with one end that is connected to
the second inlet; a connection portion configured to connect another end
of the first main pipe and another end of the second main pipe; a first
bypass pipe configured to bypass the first pump by connecting the first
inlet side of the first pump and the connection portion; and a second
bypass pipe configured to bypass the second pump by connecting the second
outlet side of the second pump and the connection portion, wherein the
first main pipe and the second bypass pipe are connected in a same
direction in the connection portion, and the second main pipe and the
first bypass pipe are connected in a same direction in the connection
portion.

2. The cooling apparatus according to claim 1, wherein an angle formed by
the first main pipe and the first bypass pipe is acute.

3. The cooling apparatus according to claim 2, wherein the connection
portion is a pipe, the first main pipe and the first bypass pipe are
connected to one end of the pipe, and the second main pipe and the second
bypass pipe are connected to another end of the pipe.

4. The cooling apparatus according to claim 2, wherein a cross-sectional
area of the pipe is equal to a sum of a cross-sectional area of the first
main pipe and a cross-sectional area of the first bypass pipe.

5. The cooling apparatus according to claim 1, wherein an inner diameter
of the first bypass pipe is equal to an inner diameter of the second main
pipe, and an inner diameter of the second bypass pipe is equal to an
inner diameter of the first main pipe.

6. The cooling apparatus according to claim 1, wherein the first main
pipe and the second bypass pipe are positioned on a same straight line,
and the second main pipe and the first bypass pipe are positioned on a
same straight line.

7. An electronic device comprising: an electronic component, and a
cooling apparatus configured to cool the electronic component, wherein
the cooling apparatus includes: a circulation path in which a coolant
circulates, where the coolant cooling the electronic component; a first
pump provided in the circulation path, the first pump including a first
inlet configured to take in the coolant and a first outlet configured to
discharge the coolant; a second pump connected to the first pump in
series in the circulation path, the second pump including a second inlet
configured to take in the coolant and a second outlet configured to
discharge the coolant; a first main pipe with one end that is connected
to the first outlet; a second main pipe with one end that is connected to
the second inlet; a connection portion configured to connect another end
of the first main pipe and another end of the second main pipe; a first
bypass pipe configured to bypass the first pump by connecting the first
inlet side of the first pump and the connection portion; and a second
bypass pipe configured to bypass the second pump by connecting the second
outlet side of the second pump and the connection portion, wherein the
first main pipe and the second bypass pipe are connected in a same
direction in the connection portion, and the second main pipe and the
first bypass pipe are connected in a same direction in the connection
portion.

Description

[0001] This application is based upon and claims the benefit of priority
of the prior Japanese Patent Application No. 2016-29719, filed on Feb.
19, 2016, the entire contents of which are incorporated herein by
reference.

FIELD

[0002] The embodiments discussed herein are related to a cooling apparatus
and an electronic device.

BACKGROUND

[0003] Air cooling and liquid cooling are used for cooling electronic
devices such as servers. The liquid cooling is a method of cooling
electronic devices using the evaporation heat and the sensible heat of
coolant, and is capable of cooling the electronic devices more
efficiently than the air cooling.

[0004] In the liquid cooling system, pumps are provided for circulating
coolant in the electronic device. When there is only one pump in the
system, the circulation of the coolant is stopped when the pump breaks
down, resulting in the situation where the electronic device cannot be
cooled. To prevent such a situation, it is effective to provide a
plurality of pumps to make the pumps redundant so that even when one of
the pumps fails, the other pumps can keep circulating the coolant.

[0005] However, the system of the redundant pumps still has room of
improvement in increasing the flow rate of the circulating coolant.

[0006] Note that techniques related to this application are disclosed in
Japanese Laid-open Patent Publications Nos. 2005-315255, 2007-103470, and
2005-228237.

SUMMARY

[0007] According to one aspect discussed herein, there is provided a
cooling apparatus including: a circulation path in which a coolant
circulates, where the coolant cooling an electronic component; a first
pump provided in the circulation path, the first pump including a first
inlet configured to take in the coolant and a first outlet configured to
discharge the coolant; a second pump connected to the first pump in
series in the circulation path, the second pump including a second inlet
configured to take in the coolant and a second outlet configured to
discharge the coolant; a first main pipe with one end that is connected
to the first outlet; a second main pipe with one end that is connected to
the second inlet; a connection portion configured to connect another end
of the first main pipe and another end of the second main pipe; a first
bypass pipe configured to bypass the first pump by connecting the first
inlet side of the first pump and the connection portion; and a second
bypass pipe configured to bypass the second pump by connecting the second
outlet side of the second pump and the connection portion, wherein the
first main pipe and the second bypass pipe are connected in a same
direction in the connection portion, and the second main pipe and the
first bypass pipe are connected in a same direction in the connection
portion.

[0008] The object and advantages of the invention will be realized and
attained by means of the elements and combinations particularly pointed
out in the claims.

[0009] It is to be understood that both the foregoing general description
and the following detailed description are exemplary and explanatory and
are not restrictive of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is a structural diagram of a cooling apparatus used in a
study;

[0011] FIG. 2 is a schematic diagram illustrating a problem of the cooling
apparatus used in the study;

[0012] FIG. 3 is a diagram schematically illustrating a structure studied
to prevent a backward flow of coolant;

[0013] FIG. 4 is a diagram schematically illustrating a situation where
the coolant flows through a circulation path;

[0014] FIG. 5 is a perspective view of a rack housing servers according to
a present embodiment;

[0015] FIG. 6 is a perspective view of a server according to the present
embodiment;

[0016] FIG. 7 is a top view of an evaporator and its surroundings
according to the present embodiment;

[0017] FIG. 8 is a side view partially illustrating a cross section of an
electronic component and the evaporator according to the present
embodiment;

[0018] FIG. 9 is a structural diagram of a cooling apparatus according to
the present embodiment;

[0019] FIG. 10A is a diagram schematically illustrating a case where both
a first pump and a second pump are working in the present embodiment;

[0020] FIG. 10B is a diagram schematically illustrating a flow of the
coolant in the case where both the first pump and the second pump are
working in the present embodiment;

[0021] FIG. 11A is a diagram schematically illustrating a case where the
first pump is stopped and the second pump is working in the present
embodiment;

[0022] FIG. 11B is a diagram schematically illustrating a flow of the
coolant in the case where the first pump is stopped and the second pump
is working in the present embodiment;

[0023] FIG. 12A is a diagram schematically illustrating a case where the
first pump is working and the second pump is stopped in the present
embodiment;

[0024] FIG. 12B is a diagram schematically illustrating a flow of the
coolant in the case where the first pump is working and the second pump
is stopped in the present embodiment;

[0025] FIG. 13 is a diagram indicating an investigation result about to
what extent the backward flow is reduced in the present embodiment;

[0026] FIG. 14 is an enlarged structural diagram of the cooling apparatus
according to a first example of the present embodiment; and

[0027] FIG. 15 is an enlarged structural diagram of the cooling apparatus
according to a second example of the present embodiment.

DESCRIPTION OF EMBODIMENTS

[0028] Prior to describing an embodiment, descriptions are provided for a
study conducted by the inventors of the present application.

[0029] FIG. 1 is a structural diagram of a cooling apparatus used in this
study.

[0030] This cooling apparatus 1 is housed in an electronic device such as
a server, and has a circulation path 2 through which coolant C
circulates.

[0031] On the circulation path 2, an evaporator 4 and a condenser 5 are
provided.

[0032] The evaporator 4 is adhered to an electronic component 3 such as a
central processing unit (CPU) which is an object to be cooled down. The
evaporator 4 receives heat from the electronic component 3 to evaporate
the coolant C, and thus cools the electronic component 3 by the
evaporation heat of the coolant C.

[0033] The condenser 5 receives air flow created by a fan 6, and thus
cools vapor of the coolant C to liquefy the coolant C.

[0034] In addition, on the circulation path 2, a first pump 11 and a
second pump 12 are provided for circulating the coolant C.

[0035] Since the two pumps 11 and 12 are provided in this manner, even in
the case where one of the pumps breaks down, the other can circulate the
coolant C, so that it is possible to prevent insufficient cooling of the
electronic component 3.

[0036] As methods of connecting the first pump 11 and the second pump 12,
there are parallel connection and series connection. In this example, the
pumps 11 and 12 are connected in series along the direction D.sub.1 in
which the coolant flows, thereby preventing the pumps 11 and 12 from
occupying a wide area in direction D.sub.2 orthogonal to direction
D.sub.1, which in turn saves the space of the electronic device.

[0037] In addition, in this example, a first bypass pipe 13 is provided to
the first pump 11, and a second bypass pipe 14 is provided to the second
pump 12. Thus, the bypass pipe 13 or 14 provides a flow passage in the
case where one of the pumps is stopped.

[0038] For example, when the first pump 11 is stopped, the coolant C flows
through the first bypass pipe 13 by the driving force of the second pump
12, so that the coolant C can keep circulating through the circulation
path 2.

[0039] However, the study conducted by the inventors revealed that this
structure has the following problem.

[0040] FIG. 2 is a schematic diagram illustrating the problem.

[0041] In the example of FIG. 2, it is assumed that the first pump 11 is
stopped and the second pump 12 is working.

[0042] In this case, the coolant C is flowing through the first bypass
pipe 13 in the right direction, in such a manner that the coolant C
bypasses the stopped first pump 11.

[0043] Meanwhile, at the downstream 12y of the working second pump 12, the
pressure of the coolant C becomes higher than that at the upstream 12x,
because of the coolant C discharged from the second pump 12. Because the
coolant C tends to flow from a part where the pressure is high to a part
where the pressure is low, a backward flow occurs in the second bypass
pipe 14, in such a manner that the coolant C flows in the opposite
direction than that in the first bypass pipe 13.

[0044] When the backward flow occurs in this manner, the flow rate of the
coolant C discharged from the second pump 12 reduces, thereby making it
difficult to efficiently cool the electronic component 3 by the coolant
C.

[0045] FIG. 3 is a diagram schematically illustrating a structure studied
to prevent the backward flow.

[0046] In the example in FIG. 3, a first check valve and a second check
valve 16 are provided on the first bypass pipe 13 and the second bypass
pipe 14, respectively. The opening and closing of these check valves 15
and 16 are controlled by a controller 19.

[0047] FIG. 4 is a diagram schematically illustrating a situation where
the coolant C flows through the circulation path 2 in this case.

[0048] In the example in FIG. 4, it is assumed that the first pump 11 is
stopped and the second pump 12 is working as in the example of FIG. 2.

[0049] In this case, by opening the first check valve 15 under the control
of the controller 19, the coolant C bypassing the first pump 11 can flow
through the first bypass pipe 13.

[0050] In addition, by closing the second check valve 16 under the control
of the controller 19, it is possible to prevent the coolant C from
flowing backward through the second bypass pipe 14.

[0051] Note that in the case where the first pump 11 is working and the
second pump 12 is stopped, the opening and closing states of the first
check valve 15 and the second check valve 16 may be reversed.

[0052] According to this structure, the backward flow of the coolant C,
which flows through the bypass pipes 13 and 14 backwardly, can be
prevented. However, the check valves 15 and 16, and the controller 19 to
control the opening and closing of these valves make the apparatus
structure more complicated.

[0053] In addition, the resistances received from the check valves 15 and
16 would reduce the flow rate of the coolant C. To compensate for the
reduction of the flow rate, the rotation speeds of the pumps 11 and have
to be increased, resulting in an increase of the electric power consumed
by the pumps 11 and 12.

[0054] Moreover, in the case where the coolant C contains foreign matters,
the foreign matter may be caught at the check valve 15 or 16, and
obstruct the flow of the coolant C.

[0055] It may be considered that the inner diameters of the bypass pipes
13 and 14 are made larger to reduce the resistance to the coolant C
received from the check valves 15 and 16. However, this approach
increases the weights of the bypass pipes 13 and 14, preventing the
weight reduction of the electronic device. In addition, there is another
problem that bending a pipe with a large inner diameter increases the
curvature radius of the bent portion of the pipe, making it difficult to
achieve downsizing of the electronic device.

[0056] In addition, in this structure, three way valves such as T-shaped
joints are used at two points to form the first bypass pipe 13 and the
second bypass pipe 14 as branch paths from the circulation path 2. Since
the T-shaped joint changes the flow direction at a right angle, the
friction loss of the coolant C is large. Hence, there is also a problem
that pressure losses becomes large at the two points of the circulation
path 2.

[0057] Hereinafter, descriptions are provided for an embodiment capable of
suppressing the backward flow in the bypass pipes without using the check
valve.

Present Embodiment

[0058] In the present embodiment, an example of cooling servers is
described.

[0059] FIG. 5 is a perspective view of a rack housing the servers.

[0060] As illustrated in FIG. 5, the rack 20 houses a plurality of
rack-mounted servers 21.

[0061] FIG. 6 is a perspective view of a server 21.

[0062] The server 21 is an example of an electronic device, and includes a
circuit board 22 and electronic components 23 such as a CPU mounted
thereon.

[0063] In this example, the circuit board 22 is provided with two
electronic components 23, and two cooling apparatuses 50 are provided for
cooling the respective electronic components 23.

[0064] Each cooling apparatus 50 includes a loop-shaped circulation path
30 in which coolant C circulates, an evaporator 24, and a condenser 27.

[0065] Among them, the evaporator 24 evaporates the coolant C with the
heat of the electronic component 23, and thus cools the electronic
component 23 with the evaporation heat of the coolant C. The coolant C is
not particularly limited. In this embodiment, pure water is used as the
coolant C, for example. Note that in order to prevent the pure water from
freezing, ethanol may be added to the pure water such that the
concentration of the ethanol becomes 0.1 wt % to 5 wt %.

[0066] A metal fixing plate 25 is mounted on the evaporator 24. By fixing
the fixing plate 25 to the circuit board 22 with screws, the electronic
component 23 is fixed to the circuit board 22.

[0067] In addition, the circulation path 30 circulates the coolant C
between the evaporator 24 and the condenser 27, and may be made by
connecting a plurality of pipes, for example. Note that the pipes
included in the circulation path 30 are simplified in FIG. 6. Details of
the pipes are described later.

[0068] In order to fill the circulation path 30 with the coolant C, the
coolant C subjected to degassing is supplied to the circulation path 30
after the circulation path 30 is evacuated to about -100 kPa.

[0069] In addition, in the circulation path 30, the first pump 11 and the
second pump 12 are connected in series for circulating the coolant C. The
two pumps provided in this manner make the pumps redundant, so that even
when one of the pumps fails and stops, the other pump can keep sending
the coolant.

[0070] The condenser 27, provided in the circulation path 30, cools and
liquefies the coolant C evaporated with the heat of the electronic
component 23. Note that fans 28 are fixed beside the condensers 27 on the
circuit board 22, and air flows created by the fans 28 promote the heat
radiation of the condensers 27.

[0071] In addition, memories 26 are mounted on the circuit board 22, which
are used for various computations in corporation with the electronic
components 23.

[0072] FIG. 7 is a top view of the evaporator 24 and its surroundings.

[0073] As illustrated in FIG. 7, a pipe 35 for liquid and a pipe 36 for
vapor are connected to the upper surface of the evaporator 24.

[0074] The pipe 35 and the pipe 36 are metal pipes which form part of the
circulation path 30. The pipe for liquid supplies the coolant C in the
liquid phase to the evaporator 24, and the pipe 36 for vapor discharges
the vapor of the coolant C from the evaporator 24.

[0075] In this example, the diameter of the pipe 36 for vapor is made
larger than that of the pipe 35 for liquid to make it easy for the vapor
of the coolant C to be pushed out from the evaporator 24 by the pressure
in the evaporator 24, making it easy for the coolant C to circulate in
the circulation path 30.

[0076] FIG. 8 is a side view partially illustrating a cross section of the
electronic component 23 and the evaporator 24.

[0077] As illustrated in FIG. 8, the electronic component 23 includes a
circuit board 41 and a semiconductor element 42 mounted thereon, and a
metal lid 43 is provided on the upper surface of the semiconductor
element 42.

[0078] The lid 43 is adhered to the circuit board 41 with adhesive seal
material 44, and the evaporator 24 described above is adhered to the
upper surface of the lid 43.

[0079] A cavity 24 is provided in the evaporator 24. The cavity 24a is
supplied with the coolant C in the liquid phase from the pipe 35, and the
semiconductor element 42 is cooled by the evaporation heat of the coolant
C. The vapor of the coolant C generated in the cavity 24a is discharged
from the pipe 36 for vapor as described above.

[0080] Note that in order to transfer the heat of the semiconductor
element 42 to the evaporator 24 rapidly, thermal interface material (TIM)
may be provided between the semiconductor element 42 and the lid 43 and
between the lid 43 and the evaporator 24.

[0081] FIG. 9 is a structural diagram of the cooling apparatus 50
according to the present embodiment.

[0082] Note that in FIG. 9, the same elements as those described in FIGS.
5 to 8 are denoted by the same reference numerals as those in these
figures, and descriptions thereof are omitted below.

[0083] As described above, the cooling apparatus 50 includes the
loop-shaped circulation path 30 and the first and second pumps 11 and 12
provided in the circulation path 30.

[0084] Of the two pumps, the first pump 11 includes a first inlet 11a
which takes in the coolant C, and a first outlet 11b which discharges the
coolant C. Then, the second pump 12 includes a second inlet 12a which
takes in the coolant C, and a second outlet 12b which discharges the
coolant C.

[0085] The first outlet 11b of the first pump 11 is connected to one end
of a first main pipe 51, and the second inlet 12a of the second pump 12
is connected to one end of a second main pipe 52.

[0086] The other ends of the first main pipe 51 and the second main pipe
52 are connected together at a connection portion P.

[0087] The configuration of the connection P is not particularly limited.
In the present embodiment, a cross joint having connection ports in the
four directions is used as the connection portion P.

[0088] In addition, in the circulation path 30, a first bypass pipe 53 is
provided to ensure a flow passage of the coolant C in case the first pump
11 stops. The first bypass pipe 53 connects the first inlet 11a side of
the first pump 11 and the connection portion P, thereby the first pump 11
is bypassed by the first bypass pipe 53.

[0089] In similar fashion, a second bypass pipe 54 is provided in the
circulation path 30 to ensure a flow passage of the coolant C in case the
second pump 12 stops. The second bypass pipe 54 connects the second
outlet 12b side of the second pump 12 and connection portion P, thereby
the second pump 12 is bypassed by the second bypass pipe 54.

[0090] In addition, in the present embodiment, the directions of the first
main pipe 51 and the second bypass pipe 54 at the connection portion P
are aligned in the same direction G.sub.1, and the directions of the
second main pipe 52 and the first bypass pipe 53 at the connection
portion P are aligned in the same direction G.sub.2.

[0091] Since the cross joint used as the connection portion P connects
each pipe substantially at one point, the first main pipe 51 and the
second bypass pipe 54 are positioned on a straight line L.sub.1, and the
second main pipe 52 and the first bypass pipe 53 are positioned on a
straight line L.sub.2.

[0092] Note that the inner diameters and the outer diameters of the first
and second main pipes 51 and 52, and the first and second bypass pipes 53
and 54 are not particularly limited. In the present embodiment, the inner
diameters of these pipes are approximately set to 4 mm to 4.35 mm and the
outer diameters of these pipes are approximately set to 6 mm to 6.35 mm.

[0093] In addition, the materials for the first and second main pipes 51
and 52, and the first and second bypass pipes 53 and 54 are also not
particularly limited. Examples of the material for the pipes include
metals such as cupper, stainless steel, and aluminum, and resin such as
fluoroplastic and polyether ether ether ketone (PEEK).

[0094] In addition, the connection angle of each pipe at the connection
portion P is also not particularly limited. In this example, angle
.theta. between the first main pipe 51 and the first bypass pipe 53 is
set to 90 degrees.

[0095] Next, descriptions are provided for operation of the cooling
apparatus 50.

[0096] Hereinafter, descriptions are provided for each of the case where
both the first pump 11 and the second pump 12 are working, and the case
where one of these pumps is stopped.

[0097] FIG. 10A is a diagram schematically illustrating the case where
both the first pump 11 and the second pump 12 are working.

[0098] FIG. 10B is a diagram schematically illustrating a flow of the
coolant C in this case. Note in FIG. 10B that the magnitude of the flow
rate of the coolant C is indicated by the thickness of the arrows, and a
thicker arrow indicates a larger flow rate of the coolant C. This is also
the case for FIGS. 11B and 12B to be described later.

[0099] As illustrated in FIG. 10B, in this case, the coolant C mainly
flows inside the first main pipe 51 and the second main pipe 52, which
directly receive the driving forces of the first pump 11 and the second
pump 12, respectively.

[0100] On the other hand, FIG. 11A is a diagram schematically illustrating
the case where the first pump 11 is stopped and the second pump 12 is
working.

[0101] Note in FIG. 11A and undermentioned FIG. 12A that "high" is affixed
where the pressure of the coolant C is high, and "low" is affixed where
the pressure of the coolant C is lower than the pressure at the portion
with "high". These words indicate a relative pressure difference in the
circulation path 30. The pressure at the outlets 11b and 12b of the
working pumps 11 and 12 is used as a reference of "high", and the
pressure at the inlets 11a and 11b is used as a reference of "low".

[0102] For example, the pressure of the coolant C is "high" at the second
outlet 12b of the second pump 12, where the coolant C immediately after
being discharged from the second pump 12 flows.

[0103] In contrast, the pressure of the coolant C is "low" at the second
inlet 12a of the second pump 12, where the coolant C immediately before
being taken in the second pump 12 flows.

[0104] FIG. 11B is a diagram schematically illustrating a flow of the
coolant C in this case.

[0105] In this case, as illustrated in FIG. 11B, the coolant C flows
through the first bypass pipe 53 to bypass the stopped first pump 11.

[0106] Here, because the first bypass pipe 53 and the second main pipe 52
are connected in the same direction at the connection portion P in the
present embodiment, the coolant C coming out of the first bypass pipe 53
does not practically receive any resistance at the connection portion P,
and most of the coolant C flows into the second main pipe 52.

[0107] As a result, even when the pressure difference of the coolant C
occurs between the second inlet 12a and the second outlet 12b as
described above, the momentum of the coolant C flowing from the first
bypass pipe 53 into the second main pipe 52 overcomes the pressure
difference, and makes it difficult for the coolant C to flow backward
through the second bypass pipe 54 in the direction of arrow A.

[0108] Especially, by making the inner diameters of the first bypass pipe
53 and the second main pipe 52 equal, the inner diameters does not change
at the connection portion P. Therefore, it is possible to guide the
coolant C smoothly from the first bypass pipe 53 into the second main
pipe 52 to prevent the backward flow of the coolant C effectively.

[0109] In addition, a check valve to prevent the backward flow does not
need to be provided at the second bypass pipe 54. Therefore, the need for
a controller to control opening and closing of the check valve is
eliminated, thereby simplifying the structure of the apparatus.

[0110] Moreover, there is no risk that the flow rate of the coolant C
flowing through the first bypass pipe reduces due to resistance received
from the check valve. Furthermore, since there is no movable part such as
the check valve, the reliability of the apparatus improves.

[0111] Meanwhile, FIG. 12A is a diagram illustrating the case where the
first pump 11 is working and the second pump 12 is stopped.

[0112] FIG. 12B is a diagram schematically illustrating a flow of the
coolant C in this case.

[0113] Also in this case, since the first main pipe 51 and the second
bypass pipe 54 are connected in the same direction at the connection
portion P as described above, the coolant C coming out of the first main
pipe 51 does not practically receive any resistance at the connection
portion P, and most of the coolant C flows into the second bypass pipe
54.

[0114] Hence, even when the pressure difference of the coolant C occurs
between the first inlet 11a and the first outlet 11b, the momentum of the
coolant C flowing from the first main pipe 51 into the second bypass pipe
54 overcomes the pressure difference, and makes it difficult for the
coolant C to flow backward through the first bypass pipe 53 in the
direction of arrow B.

[0115] Note that the inner diameters of the second bypass pipe 54 and the
first main pipe 51 may be made equal to guide the coolant C smoothly from
the first main pipe 51 into the second bypass pipe 54, thereby preventing
the backward flow of the coolant C effectively.

[0116] The inventors of the present application investigated to what
extent the backward flow is reduced in the present embodiment.

[0117] The investigation result is indicated in FIG. 13.

[0118] As the investigation method, such an approach was employed which
measures the total flow rate of the coolant C flowing through the
circulation path 30, and determines based on the measured value whether
or not a backward flow is occurring in the bypass pipes.

[0119] In this investigation, a comparative example is also investigated,
where the bypass pipes 13 and 14 are connected as in FIG. 1.

[0120] Note that in FIG. 13, "normal operation" means the state where both
the first pump 11 and the second pump 12 are working. In this "normal
operation", outputs of the both of the first pump 11 and the second pump
12 were set to 100%.

[0121] In contrast, "one pump stopped" means the state where the first
pump 11 is working and the second pump 12 is stopped. In this case, the
output of the first pump 11 was set to 100%, and the output of the second
pump 12 was set to 0%.

[0122] In addition, all the outer diameters of the first and second main
pipes 51 and 52, and the first and second bypass pipes 53 and 54
according to the present embodiment were set to 6.35 mm, and the inner
diameters of these pipes were set to 4.35 mm. Note that the sizes of the
first and second bypass pipes 13 and 14 according to the comparative
example were set to the same values as these.

[0123] As indicated in FIG. 13, in the comparative example, the total flow
rate of the coolant C decreases by about 33% when one pump stops as
compared to the case of the normal operation. In contrast, in the present
embodiment, the decrease is suppressed to 26%, which is lower than 7% of
the comparative example.

[0124] The decrease in the total flow rate is caused since the second pump
12 is stopped, and also the backward flow is caused in the bypass pipe.
Therefore, the reduction of the decrease in the total flow rate in the
present embodiment than the comparative example indicates that backward
flow of the coolant C in the second bypass pipe 54 is suppressed.

[0125] From this result, it was confirmed that connecting the first main
pipe 51 and the second bypass pipe 54 in the same direction and
connecting the second main pipe 52 and the first bypass pipe 53 in the
same direction at the connection portion P as in the present embodiment
are effective to reduce the backward flow of the coolant C.

[0126] Moreover, it was found that by preventing the backward flow in this
manner, the total flow rate (390 ml/min) with one pump stopped in the
present embodiment becomes larger than the total flow rate (310 ml/min)
with one pump stopped in the comparative example.

[0127] Note that it was also found that not only in the operation with one
pump stopped but also in the normal operation, the total flow rate (530
ml/min) in the present embodiment becomes higher than the total flow rate
(460 ml/min) in the comparative example.

[0128] Next, descriptions are provided for various modification examples
of the present embodiment.

First Example

[0129] FIG. 14 is an enlarged structural diagram of the cooling apparatus
50 according to a first example.

[0130] In this example, as illustrated in FIG. 14, by setting the
aforementioned angle .theta. to 0 to 90 degrees, angle 59 formed by the
first main pipe 51 and the first bypass pipe 53 is made acute.

[0131] Since the first bypass pipe 53 bypasses the first pump 11 as
described above, a parallel portion 53x, which is parallel to sending
direction D.sub.1 in which the first pump 11 sends out the coolant C,
inevitably exists in the first bypass pipe 53.

[0132] In order to provide the parallel portion 53x, a bent portion 53y
may be provided in the first bypass pipe 53 that extends from the
connection portion P.

[0133] In this example, since the angle .theta. was set to 0 to 90 degrees
as described above, bending angle .phi. of the bent portion 53y can be
made obtuse, which makes it easy to form the bent portion 53y by bending
the first bypass pipe 53.

Second Example

[0134] FIG. 15 is an enlarged structural diagram of the cooling apparatus
50 according to a second example.

[0135] In this example, as illustrated in FIG. 15, the angle 59 between
the first main pipe 51 and the first bypass pipe 53 is made acute as in
the first example, and a pipe 60 is provided as the connection portion P.
Note that angle 61 between the second main pipe 52 and the second bypass
pipe 54 is also made acute.

[0136] Then, the first main pipe 51 and the first bypass pipe 53 are
connected to one end 60a of the pipe 60, and the second main pipe 52 and
the second bypass pipe 54 are connected to the other end 60b of the pipe
60.

[0137] Also, in this example, the first main pipe 51 and the second bypass
pipe 54 are aligned in the same direction G.sub.1 at the connection
portion P, and the second main pipe 52 and the first bypass pipe 53 are
aligned in the same direction G.sub.2 at the connection portion P.

[0138] The investigation conducted by the inventors of the present
application revealed that even when the pipe 60 is provided as the
connection portion P in this manner, the backward flow of the coolant C
in the first bypass pipe 53 and the second bypass pipe 54 can be reduced.

[0139] The reason is considered as follows.

[0140] For example, assume the case where the first pump 11 is working and
the second pump 12 is stopped.

[0141] In this case, even when the coolant C tries to flow backward into
the first bypass pipe 53 as indicated by arrow E, the acute angle 59 is
difficult for the coolant C to turn, making it difficult for the coolant
C to flow backward into the first bypass pipe 53.

[0142] In similar fashion, in the case where the first pump 11 is stopped
and the second pump 12 is working, the acute angle 61 is difficult for
the coolant C to turn, making it difficult for the coolant C to flow
backward into the second bypass pipe 54 as indicated by arrow F.

[0143] Note that although the cross-sectional area of the pipe 60 is not
particularly limited, it is preferable that the cross-sectional area of
the pipe 60 be equal to the sum of the cross-sectional area of the first
main pipe 51 and the cross-sectional area of the first bypass pipe 53.
This makes it difficult for the coolant C flowing from each of the first
main pipe 51 and the first bypass pipe 53 into the pipe 60 to receive
resistance caused by the difference in the cross-sectional area, and
allows the coolant C to flow smoothly.

[0144] Although descriptions have been provided for the present embodiment
as above, the embodiment is not limited to the above.

[0145] For example, although in the above, evaporation heat absorbed in
the phase transition of the coolant C from the liquid phase to the vapor
phase is used to cool the electronic component 23, the electronic
component 23 may be cooled using sensible heat of the coolant C in the
liquid phase without evaporating the coolant C as described above.

[0146] All examples and conditional language recited herein are intended
for the pedagogical purposes of aiding the reader in understanding the
invention and the concepts contributed by the inventor to further the
art, and are not to be construed as limitations to such specifically
recited examples and conditions, nor does the organization of such
examples in the specification relate to a showing of the superiority and
inferiority of the invention. Although one or more embodiments of the
present invention have been described in detail, it should be understood
that the various changes, substitutions, and alterations could be made
hereto without departing from the spirit and scope of the invention.